TerraPhoenix/Achievement:Commando

You can easily obtain this achievement in a quick play-through of the game. This is recommended so when you get it you can continue your game with all soldiers at somewhat equal levels. However, for the fastest and easiest way, play through Act I with only one soldier shooting. If your soldier dies or is unconscious very early, start over.

Tips:

Use other soldiers as meat-shields, allowing them to take damage while your 'chosen-one' stays alive'. A good tactic is the Gate Tactic which looks like this: